You would need to focus on what kind of work you would expect to be doing. The four main areas that immediately come to mind (at least in my experience) are teaching, research, admin, and texbook writing.

A Cambridge DELTA is the obvious extension to a CELTA and it would certainly be less expensive and less time consuming than an MA. The DELTA would put you in good stead for a job as a TESOL trainer on one of the genuine courses such as CELTA, or as a Director of Studies in a high street cram school`or large language institute.

An MA TESOL would probably be of more use than an MA in Applied LInguistics if you want to stay on the teaching side of languages . It would give you access to senior posts in mainstream education, especially if you already have a PGCE or equivalent. If you have a degree already, especially in a language or linguistics related subject, the PGCE would come somewhere between the DELTA and the MA TESOL at the higher end of the scale.

Applied Linguistics is great for jobs in research into language with relevance to real world problems. It is generally a multidisciplinary approach to research on language related concerns in various fields such as the sciences of language acquisition, lexicography, marketing, psychology, anthropology, sociology, and of course, education.

Choose carefully, because some jobs are better paid than others and you will need to envisage the potential for return on your investment.

Last edited by systematic on 19 Aug 2010, 07:08, edited 2 times in total.
Reason:another typo!

BTW, our Alex Case made this interesting post this on another thread a while back:

A friend of mine is taking the Cambridge DELTA two years after she took the CELTA and then got her first teaching job. Just the fact that she is taking the DELTA has already got her a promotion, and in the chain she works for in a year or two she could be in a position where they pay large relocation allowances and international school fees for her kids. The only other qualification that school accepts is the Trinity Cert TESOL, and pre-cert experience counts not one jot towards position on the pay scale, promotion etc. In fact, the box on the application form simply says "post qualification teaching experience" and there is nowhere you can even write experience before your cert.

The above is an extreme example, but if you want to work for IH or Bell rather than EF or Wall Street (and trust me, you do, even though the package is not quite what I described above), then the same things are basically true. By reputation the school I worked for in Japan is somewhere between those two divisions I just explained, and you could get a job without a Cert. However, with a Cert you could be Senior Teacher in two years and DoS in four. If you didn't have a Cert and wanted to climb the slippery slope, you'd need to take a month off work and go and do it sooner or later.